1. Field of the Invention
The present invention relates to a differential pumping seal apparatus for use as a part of a semiconductor fabrication apparatus or the like, and more particularly to an improved differential pumping seal apparatus having a seal element referred to as a differential pumping seal for sealing a gap between a space defined by a fixed member and a movable body movable with respect to the fixed member, and another space having an atmosphere which is different from an atmosphere in the above space with respect to a pressure, cleanliness, type of a gas, or the like in a non-contact manner.
2. Description of the Related Art
A differential pumping seal is disclosed in U.S. Pat. No. 4,118,042 entitled “Air Bearing Vacuum Seal Assembly”, filed in 1977, and U.S. Pat. No. 4,191,385 entitled “Vacuum sealed Gas bearing Assembly”, filed in 1979. A differential pumping seal used in a semiconductor fabrication apparatus is disclosed in U.S. Pat. No. 4,425,508 entitled “Electron beam Lithographic Apparatus”, filed in 1982.
The conventional differential pumping seal has been used for moving an object quickly and smoothly in a rotary-motion manner or in a linear-motion manner within a clean space such as a vacuum space without contaminating the clean space.
There are at least two cases in which the differential pumping seal is used. In the first case, the differential pumping seal is used in the case where an actuator for moving the object, a guide mechanism for guiding the object, and other components are placed outside of the clean space, and only a minimum structural assembly and the object such as a specimen are adapted to be disposed in the clean space. In the second case, the differential pumping seal is used in the case where a hydrostatic fluid bearing such as an air bearing is used for sealing two spaces from each other without impairing the advantage of a non-contact bearing.
Specifically, in the second case, the differential pumping seal is used for sealing the two spaces from each other in the following situation:
1) The bearing is placed in a clean space, and a space in which a fluid used for the bearing is present and the clean space are sealed from each other.
2) The bearing is placed outside of a clean space, and the clean space and the exterior space of the clean space are sealed from each other.
The differential pumping seal is capable of separating two spaces from each other in a non-contact fashion. This feature is recognized as a great advantage, and the differential pumping seal tends to be applied to practical apparatuses. When the differential pumping seal is in a normal or steady state in which the differential pumping seal functions normally, the differential pumping seal can certainly perform an excellent function as a non-contact seal. However, from a total viewpoint in terms of the reliability and service life of the differential pumping seal, and also cleanliness, it is necessary to sufficiently recognize the effects (features) of the differential pumping seal when the differential pumping seal is in an unsteady state in which the differential pumping seal does not function normally.
When an apparatus having a differential pumping seal is in operation, the failure of a continuous sealing function may occur due to an emergency shutdown of the apparatus, or the entire apparatus including the differential pumping seal may be brought out of operation. When such cases occur, the differential pumping seal has a disadvantage resulted from a non-contact seal. This disadvantage can easily be imagined because an invisible and virtual wall which has separated the two spaces from each other is eliminated when the sealing function is stopped. For example, when operation of the differential pumping seal which has separated a clean space (vacuum space) and a space (the atmosphere) outside of the clean space from each other in a non-contact manner is suddenly stopped, the vacuum space which has been kept clean is instantly filled with air of the atmosphere.
Normally, a substance which has a greatest effect on the degree of vacuum is water, which is typical of moisture of the atmosphere. The humidity in a clean room is normally in the range of about 40 to 55%, and thus a gas in the clean room contains a large quantity of moisture. If air of the atmosphere flows into the clean room each time the differential pumping seal stops its sealing function, then it is difficult to increase the degree of vacuum in a vacuum chamber which is to be cleaned. Further, there is a possibility that particles in the atmosphere outside of the vacuum chamber enter the vacuum chamber. In an apparatus having an ordinary vacuum chamber, no atmosphere is introduced into the vacuum chamber, and the humidity of a gas which is introduced into the vacuum chamber is controlled at an extremely low degree.
When the differential pumping seal stops its sealing function in a non-contact manner, the vacuum chamber is the same as a vacuum chamber whose partition wall for separating the vacuum chamber from the exterior has a large crack. Therefore, the differential pumping seal is, so to speak, a troublesome apparatus from the standpoint of the commonsense in the field of the conventional vacuum apparatus.
As described above, the conventional differential pumping seal (non-contact seal) has an operational disadvantage in an apparatus incorporating the differential pumping seal such as a vacuum apparatus which requires a cleanliness of an atmosphere.
In order to apply a non-contact seal to a semiconductor fabrication apparatus in practical use, it is necessary to compensate for the above operational disadvantage by adding other element.